It is known that grain oriented electrical steel sheets having crystal grains in accord with the {110}<001> orientation (hereinafter “Goss orientation”) through secondary recrystallization annealing exhibit excellent magnetic properties (see, e.g. JPS40-15644B (PTL1)).
As indices of magnetic properties, the magnetic flux density B8 at a magnetic field strength of 800 A/m and the iron loss W17/50 per kg of the steel sheet when it is magnetized to 1.7 T in an alternating magnetic field at an excitation frequency of 50 Hz, are mainly used.
One means for reducing iron loss in grain oriented electrical steel sheets is to highly accord crystal grains after secondary recrystallization annealing with the Goss orientation. In order to increase the degree to which grains are accorded with the Goss orientation after secondary recrystallization annealing, it is important to induce differences of grain boundary mobility so that only highly Goss-orientated grains preferentially grow. In detail, it is important to form a predetermined microstructure in the texture of the primary recrystallized sheet, and to use precipitates called inhibitors to suppress growth of recrystallized grains other than Goss-oriented grains.
Known examples of predetermined primary recrystallized microstructures which allow only highly Goss-orientated grains to preferentially grow include {554}<225> oriented grains and {12 4 1}<014> oriented grains. By highly according these grains in a well balanced manner in the matrix of the primary recrystallized sheet, Goss-oriented grains may be highly accorded after secondary recrystallization annealing.
For example, JP2001-60505A (PTL2) discloses that a steel sheet subjected to secondary recrystallization annealing that stably exhibits excellent magnetic properties can be obtained when the steel sheet subjected to primary recrystallization annealing possesses: a texture in the vicinity of a surface layer of the steel sheet, having a maximum orientation within 10° from either the orientation of (ϕ1=0°, Φ=15°, and ϕ2=0°) or the orientation of (ϕ1=5°, Φ=20°, and ϕ2=70°) in Bunge's Eulerian angle representation; and a texture of a center layer of the steel sheet, having a maximum orientation within 5° from the orientation of (ϕ1=90°, Φ=60°, and ϕ2=45°) in Bunge's Eulerian angle representation.
As techniques of using an inhibitor, for example, PTL1 discloses a method of using AlN and MnS, JPS51-13469B (PTL3) discloses a method of using MnS and MnSe, and both methods have been put into practical use.
These methods using an inhibitor ideally require a uniform and fine precipitate distribution of the inhibitor, and in order to achieve such state, it is necessary for the slab heating before hot rolling to be performed at a high temperature of 1300° C. or higher. However, as such high temperature slab heating is performed, the crystal structure of the slab becomes excessively coarse. The orientation of the slab structure is mostly a {100}<011> orientation which is a stable orientation of hot rolling, and such coarsening of the slab structure greatly impedes secondary recrystallization, and causes a significant deterioration of magnetic properties. Therefore, for grain-oriented electrical steel sheets obtained using an inhibitor and performing high temperature slab heating, it is necessary to contain C of around 0.03% to 0.08% in the material for the purpose of using the α-γ transformation during hot rolling to break the coarse slab structure. Nevertheless, if C remains in the product steel sheet, the magnetic properties of the product steel sheet are significantly deteriorated. Therefore, it is also necessary to perform decarburization annealing in any step after hot rolling to reduce the C content in the product steel sheet to around 0.003% or less.
As described above, in conventional methods of producing grain-oriented electrical steel sheets by using an inhibitor, high temperature slab heating requires a large energy, and a decarburization annealing step needs to be provided. Therefore, manufacturing costs are increased.
To address this issue, for example, JPH5-112827A (PTL4) discloses a so-called nitriding treatment technique in which magnetic properties equivalent to that achieved by high temperature slab heating can be achieved by performing low temperature slab heating. To achieve said purpose, the slab heating temperature is set to a low temperature of 1200° C. or lower, and in the slab heating stage, inhibitor forming elements such as Al, N, Mn, S are not completely dissolved in steel. After decarburization annealing, annealing is performed in a strongly-reductive atmosphere such as a mixed atmosphere of NH3 and H2 while running the steel sheet, to form an inhibitor mainly composed of (Al,Si)N.
Further, JPS57-114614A (PTL5) discloses a method of subjecting a silicon steel slab containing 0.02% or less of C to rough hot rolling at a starting temperature of 1250° C. or lower to obtain a hot rolled sheet, then subjecting the hot rolled sheet to recrystallization hot rolling in which the cumulative rolling reduction at 900° C. or higher is 80% or more and at least one pass applies a rolling reduction ratio of 35% or more, and then subjecting the hot rolled sheet to strain accumulating rolling in which the cumulative rolling reduction at 900° C. or lower is 40% or more, to break the slab structure even in steel with low C material.
However, in this method, although inhibitor elements such as Al and N are contained in steel, high temperature slab heating is not performed, and therefore, fine precipitation of the inhibitor does not occur. Further, since nitriding treatment such as mentioned above is not performed, the growth inhibiting effect of primary recrystallized grains is insufficient and magnetic properties deteriorate. In addition, cooling conditions before final cold rolling and after annealing are not specified, and contents of solute elements (C, N and the like) are not sufficiently controlled.
JPH6-346147A (PTL6) discloses a method of subjecting a silicon steel slab containing 0.0005% to 0.004% of C to rough hot rolling at a starting temperature range of 1000° C. to 1200° C. to obtain a hot rolled sheet, and then subjecting the hot rolled sheet to short time annealing in a temperature range of 700° C. to 1100° C. as necessary, and subsequent cold rolling once, or twice or more with intermediate annealing performed therebetween to obtain a cold rolled sheet, then heating the cold rolled sheet in a temperature range of 850° C. to 1050° C. for 1 second or more and 200 seconds or less, and then subjecting the steel sheet to nitriding treatment while running the steel sheet. However, as in the case with the method of PTL5, although inhibitor elements such as Al and N are contained in steel, high temperature slab heating is not performed, and therefore, fine precipitation of the inhibitor is insufficient. Accordingly, the growth inhibiting effect of primary recrystallized grains is insufficient and magnetic properties deteriorate. In addition, cooling conditions before final cold rolling and after annealing are not specified, and contents of solute elements (C, N and the like) are not sufficiently controlled.